1. Field of the Invention
The present invention generally relates to the conversion of chemical energy to electrical energy, and more particularly, to a nonaqueous electrolyte activating a rechargeable alkali metal electrochemical cell, particularly a lithium ion cell, designed for long cycle life and low temperature charge/discharge applications.
2. Prior Art
Lithium ion cells activated with carbonate based electrolytes are known to deliver more than 1,000 charge/discharge cycles at room temperature. However, the poor discharge or cycling behavior of such cells at low temperatures, i.e., temperatures &lt;-20.degree. C., remains a vexing challenge with this technology. One of the biggest contributors to this problem is the poor conductivity of typically used organic carbonate electrolyte systems which freeze at temperatures below -10.degree. C. For example, an electrolyte consisting of 1.0M LiPF.sub.6 dissolved in a mixture, by volume, of EC:DMC=30:70 has a freezing point of -11.degree. C. (see electrolyte 9 in Example II); and electrolytes consisting of 1.39M LiPF.sub.6 dissolved in a mixture, by volume, of EC:DEC=50:50 to 25:75 have freezing points higher than -15.degree. C. (as reported by S. T. Mayer et al.). As a result, lithium ion cells activated with binary carbonate electrolytes can not be effectively charge/discharge cycled at temperatures below -15.degree. C.
One solution to this problem is to provide an electrolyte containing organic solvent constituents in addition to the carbonates. For example, lithium ion cells activated with electrolytes containing an organic ester co-solvent are known to exhibit good low temperature discharge performance, (S. T. Mayer, H. C. Yoon, C. Bragg, and J. H. Lee of PolyStor Corporation, presentation at The Thirteenth International Seminar on Primary and Secondary Battery Technology and Application, Mar. 4-7, 1996, Boca Raton, Fla.; E. J. Plichta, S. Slane, Journal of Power Sources 69, 41-45, 1997; Y. Ein-Eli, S. R. Thomas, R. Chadha, T. J. Blakley, and V. R. Koch, J. Electrochem. Soc., 144, 823-829, 1997). Further, U.S. Pat. No. 5,256,504 to Okuno et al. describes a mixed solvent electrolyte of ethylene carbonate (EC) and the esters methyl acetate and methyl propionate for activating lithium ion cells. This solvent system exhibits improved low temperature (-10.degree. C.) discharge performance relative to cells activated with electrolytes composed of an EC and DEC mixed solvent system. Many other researchers also report similar observations when an organic ester solvent is used as a co-solvent with an organic carbonate. However, while mixed carbonate and ester solvent system electrolytes present higher conductivity at low temperatures than conventional carbonate based electrolytes, organic esters are known to have generally low melting points and low viscosities.
Furthermore, while lithium ion cells activated with electrolytes containing an organic ester as a co-solvent are known to exhibit improved low temperature discharge performance, this is achieved at the expense of the cell's cycle life. When cycled at room temperature, lithium ion cells activated with an electrolyte containing an organic ester as a co-solvent exhibit significantly higher capacity fade than cells having a similar electrode couple activated with an electrolyte containing only organic carbonate solvents. This detrimental effect is probably due to the oxidative instability of organic esters in the presence of high voltage cathode materials.
According to the present invention, low temperature discharge and capacity fade problems are avoided by activating an alkali metal rechargeable cell, and particularly a lithium ion cell, with a quaternary mixture of organic carbonate solvents in the electrolyte. The carbonate solvents comprising the quaternary mixture and the solvent mixture ratio are carefully selected to provide a lithium ion cell capable of discharge at temperatures below -20.degree. C. and that exhibits good cycling characteristics. At the same time, the cycleability of the present invention cells at room temperature is maintained as good as rechargeable cells of a similar electrode chemistry activated with the conventional binary carbonate solvent electrolytes.